configure.ac : Add -Wmissing-declarations to CFLAGS.
src/sd2.c : Fix segfault in SD2 RSRC parser.

A specially crafted resource fork for an SD2 file can cause
the SD2 RSRC parser to read data from outside a dynamically
defined buffer. The data that is read is converted into a
short or int and used during further processing.

Since no write occurs, this is unlikely to be exploitable.

Bug reported by The Mayhem Team from Cylab, Carnegie Mellon
Univeristy. Paper is:
http://users.ece.cmu.edu/~arebert/papers/mayhem-oakland-12.pdf